New and advanced continuous torque transmission systems, having continuous slip torque converters and shifting clutch systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, the friction materials technology must be also developed to meet the increasing energy requirements of these advanced systems.
In particular, a new high performance, durable friction material is needed. The new friction material must be able to withstand high speeds wherein surface speeds are up to about 65 m/seconds. Also the friction material must be able to withstand high facing lining pressures up to about 1500 psi. it is also important that the friction material be useful under limited lubrication conditions.
The friction material must be durable and have high heat resistance in order to be useful in the advanced systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is being generated during operating conditions.
The high speeds generated during engagement and disengagement of the new systems mean that a friction material must be able to maintain a relatively constant friction throughout the engagement. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize “shuddering” of materials during braking or the transmission system during power shift from one gear to another. It is also important that the friction material have a desired toque curve shape so that during frictional engagement the friction materials is noise or “squawk” free.
While phenolic resins are conventionally used as an impregnant in wet friction materials for wet clutch applications, the phenolic resins have various limitations. The phenolic resin friction materials do not have the high heat resistance necessary for use with the new high energy transmission systems. In particular, the phenolic resins decompose at a temperature of about 300° C. which is too low to be useful in high energy applications. In addition, phenolic resins are rigid materials and when the phenolic resins are used in a friction material, uneven lining wear, and separator plate “hot spots” are more likely result if uniform contact is not obtained.
As a result of extensive research in view of the need for a better friction material, a friction material with improved characteristics has been developed by the invention. The present wet friction material is useful in “wet” applications where the friction material is “wetted” or impregnated with a liquid such as brake fluid or automatic transmission fluid during use. During use of the “wet” friction material, the fluid is ultimately squeezed from or is impregnating the friction material. Wet friction materials differ greatly, both in their compositions and physical characteristics from “dry” friction materials.
A need exists to continue the search for increased porosity of “wet” friction material. The open pores of existing “wet” friction material made by the paper making process is about 50 percent by volume. The porosity of other “wet” friction materials such as those made with compression molding is below 50 percent by volume.